Electron decelerator



March 27, 1945. R. F. BAKER 2,372,170

ELECTRON DECELERATOR Filed March 23. 1942 Pars/WML -A K wmfdlvalr attorney Patented Mar. 27, `1945 lELECTRON DECELERATOR .Richard.F.Baker', Merchantvlle, N. J., assignor to Radio Corporation of America, a corporation Iof 'Delaware Application Marchv23, 1942, Serial No. 435,827

(Cl. Z50-49.5)

11 Claims.

"Ihisinventionirelatesgenerally to electron decelerators'andparticularly to methods of and apparatus for deceleratingand difusingelectrons in electronfmicroscopes for Ythe purpose of obtaining optimum electron velocity nforti-1e exposure of sensitized .emulsions :or for y-lormlng electronic images.

It vhas been customary to accelerate the lelectrons irradiatin'g the object in vIan "electron microscope to velocities vat whichfthe electrons pierce the object. lIn many instances, such `-velo`cities require accelerating'voltagesinlexcess of :100 kilovolts. vExperiments indicate that such velocities are vinetiicientffor exposureofphotographic emulsions, since Vthe loptimum velocity for :this purpose r'is produced by 4accelerating voltages of approximately 180 to 1100 kilovolts, :depending `on the emulsion characteristics. velocities substantially abovepr vbelow this -eritical value -produce less effect-upon:photographic-emulsions:and

therefore `'require longer exposure `for V`a` vgiven image density. 4lhereiore itis'desirable toirradiate the object or otherwise utilize the electron beam at'the'm'ost advantageous velocity and then alter the velocity tothe optimum value lfor ex- Dosure of `the sensitized emulsion.

AAn object of -this invention lis '-to provide la method oi 'and means ror-'chang'ing `the velocity oi an electron beam -in anfel'ectron' microscopeor 4other electronicdevice prior fto 'the fimpingement of the electrons on'afphotographically sensitized emulsion. Another obj ect is -to provide 'fmeans for changing the velocityl 'Of'electrons `to the optimum value required tor-exposure v'offa sensitized emulsion or vfor `forming electronic images. -Still v-another object is to provide in yan electron micro.

scope, or similar-"electronicdevicefa velocity controlling device Tor altering the velocity of electronsimpinging on the sensitized-emulsion-of a photographic target. Another object is to provide a screen "having desired jelectron 'scattering -or diffusion characteristics.

The 'invention will '^be"described fwith reference tothe 'drawing of fwl'ii'c'xh Figure l1 lis a `cross-sectional View 'of a "supportcontain'ing la sensitized 'emulsion and anfelectron deceleratom Vfigure v2 is a schematic diagram of an-'lectron'decelerator in combinationwith'typical-electronic apparatus; Figurev 3 is Va schematic Adiagram 'or another embodiment of 'the'invention and Figure-4 is -a kgraph of fthe electron sensitivity characteristics of a typical vphotographic emulsion. Similar `refer- Sencenumer'als"are'applied to vsimilar' Aelements throughoutthedrawing.

' Y'Referring'to"l3ig."1,"a base i containing'a'photofis graphic `emulsion 12 which is 'sensitive Ato electron bombardment isshown in the Aield of an electron beam 4. An `electron decelerato'r 3, whichmay consists vo'f lone or more vthin layers :of Ymetal foil, oil, paint, or suitable vcellulous material, is located 'between lthe sensitizedemulsion and the source of the .electron beam. The decelerator material should vbe of high atomic '-number, Aand should be located adjacent to lor'in contact with the photographic emulsion layer. The plural layers may be in intimate contact or separated a predetennineddistance.

Fig. -2 fis `any `electronic device in which an electron v`beam is caused to expose a photographic emulsion. .An electron source VIl) is supplied with,l vpower from a 'battery nor other source or 'voltage l. 'Electrons emittedby lthe source vlll-are ydrawn toward an accelerating electrode 6 supplied with a high positive potential from the source v8. The 'accelerating electrode causes Vthe electron beam topenetrate and pass-through the object or specimen i3. An electrostatic or magnetic 'electron focusing means 9 causes the electron beam to Vbe focused upon `the photographically'sensitized `surface 2 of the `base l. Since the velocity `of fthe 'electrons 4 'required for penetration of the object I3 is considerably higher than theoptlmum electron Velocity for exposure'o'fth'ephotographic emulsion, one `or more layers 3 `of an yelectron permeable material which vmay be Imetal foil, oil, paint, cellulose 'or the like is interposed between the objective `and -the Asensitized emulsion -to reduce the 4average velocity "of the electrons in the electron beam tothe optimumvalua-and cause diffusion `of `therele'ctrons 'throughout the emulsion. The decelerator 3 `should Vbe located as close as possible vto the 'emulsion vlayer V2 'if goed 'The 'de'celerating Ael'ectrode'i-l dshould vvbe at suit- T able negative potential to decelerate the-electron beam to an optimumyelocity. This embodiment will permit 4desired electron ldeceleration -'with out causing'undesired `electron 'scattering lor diffusion `inhe'rentin the decelerating umeans of Fig. 2. Freedom from electron scattering is particularly important wh'enelectron'irnages of high definition 'are desired. If some ldegree 'ofelectron scattering is found advantageous in particular apparatus, this may be readily accomplished by the addition of a thin metal foil or grid element I4 connected to, and preferably located inside, the electrode Il. The characteristics of the foil or grid will determine the degree of scattering and will also contribute to the deceleration. It should be understoodthat the electrode ll also can be used for accelerating the electron beam to optimum velocity by connecting it to a suitable source of positive potential ini stead of the negative potential I2. This embodiment per se is claimed in application Serial No. 435,788, led March 23, 1942, by James L. Whittaker and assigned to the same assignee.

Fig. 4 is a graph S which indicates the sensitivity of a typical photographic emulsion to electron velocities produced by accelerating voltages from to 300 kilovolts. It is apparent that the emulsion sensitivity commences at about 3 kilovolts and continues to rise to a maximum at approximately 80 kilovolts after which it drops to a relatively low value as the accelerating voltage is increased. The optimum value of voltage varies somewhat with emulsion characteristics. The decelerator means, should be of predetermined thickness, depending on the material used, to reduce the electron velocity to the optimum value.

It will be seen that the means which have been described operate on an entirely different principle than typical optical lters, in that this invention controls the velocity and diffusion of the electrons impinging on a photographic emulsion while optical iilters control either the frequency or the intensity of the light beam applied to the if emulsion. Experiments indicate that the eifect described is probably not due to secondary emission in the velocity control element or to fiuores ence or radiation from the decelerator or the photographic emulsion. While no attempt has been made to explain the reasons for the relation between photographic emulsion sensitivity and electron velocity, experimental data clearly shows that the relation exists, and that electron velocity can be controlled for optimum emulsion efficiency by the means and methods disclosed as by this invention. y

Iclaim as my invention:

V1.c In electronic apparatus including an evacuated chamber, a photographically sensitized electron target disposed within said evacuated chamber, electron velocity changing means and electron diffusing means disposed in the electron path adjacent said target, the method comprising directing said electrons at said target, changing the velocity of said electrons to an optimum value for the exposure of said target and diffusing said electrons throughout the sensitized portion of said target.

2. In an electron 4microscope, including an evacuated chamber, an object and a photographically sensitized target disposed Awithin said chamber, the method comprising accelerating said electrons to penetrate said object, focusing the electron image of said object on said target, and altering the velocity of the electrons of said image to the optimum value for the exposure of said target.

' 3. In electronic apparatus including an evacuated chamber, a source of electrons, means for accelerating said electrons, electron Y focusing means, an object, `means for decelerating said electrons, and a photographically sensitized target disposed within said chamber, the method comprising accelerating saidlelectrons, focusing said electrons on said object to form an image thereof, directing said image at said target, and

changing the velocity of said electrons betweensaid object and target to the optimum value for the exposure of said second target.

4. In electronic apparatus including an evacuated chamber, a source of electrons, means for accelerating said electrons, a photographically sensitized target'disposed within said chamber and means for directing said electrons at said target, means disposed between said source and said target comprising at least one thickness of electron permeable material to decelerate substantially all of said electrons an amount to provide optimum photographic exposure of said target.

5. In electronic apparatus including an evacuated chamber, a vsource `of electrons, means for accelerating said electrons, a photographically sensitized target disposed within said chamber and means for directing said electrons at said target, means disposed between said source and said target comprising at least one thickness of a dense material to decelerate substantially all of said electrons an amount to provide optimum photographic exposure of said target.

6. In electronic apparatus including an evacuated chamber, a source of electrons, means for accelerating said electrons, a .photographically sensitized target disposed within said chamber and means for directing said electrons at said target, means disposed between said source and said target comprising at least one thickness of a metallic foil to decelerate substantially all of said electrons an amount to `provide optimum photographic exposure of said target. l ,v

7. In electronic apparatus including an evacuated chamber, a source of electrons, means for vaccelerating said electrons, a photographically sensitized target disposed within said chamber and means for directing said electrons at said target, means disposed between said source and said target comprising at least one thickness of a cellulose material to decelerate substantially all of said electrons an amount to -provide loptimum photographic exposure of said target. i

8. In electronic apparatus including an evacuated chamber, a .source o f electrons, means for accelerating said electrons, a photographically sensitized target disposedwithin said chamber vand means for directing said electrons at said target, means disposed between said source and said target comprising at least one thickness of electron permeable material to decelerate substantially all of said electrons to a value which is a function of the thickness of saidmaterial for optimum exposure of said target. y

9.V In electronic apparatus including an evacuated chamber, a source of electrons, means for accelerating said electrons, a photographically sensitized target disposed within said chamber and means for directing said electrons at said target, means disposed betweenfsaid source andsaid target comprising at least one thickness of a dense material to decelerate substantially all of said electrons to a Value which'is a function of the thickness of vsaid material for a predetermined exposure of saidtarget. 1 s Y 10. In electronic apparatus including an evacuated chamber, a source ofelectrons, means for accelerating saidv electrons, a photographically sensitized target disposedvwithin said chamber and means vfor directingA said electrons at said target, means disposed between `said 'source and said'target comprising at least one thickness of a metallic foil to deceleratesubstantially .all of said electrons to a value which is a function of the thickness of said foil for optimum exposure of said target.

11. In electronic apparatus including an evacuated chamber, a source of electrons. `means for accelerating said electrons, means disposed within said chamber for supporting a photographically sensitized target and means for directing said electrons at said target, means disposed between said source and said target comprising at least one thickness of a cellulose material to de- RICHARD F. BAKER. 

